Development of a two-layer recording-reproduction optical disk wherein a recording film of two layers is provided on one side of an optical disk to increase the recording capacity per one side to twice is proceeding.
In the two-layer recording-reproduction optical disk, as shown in a sectional view of FIG. 1, a recording film L1 layer for recording data, a space layer, another recording film L0 layer for recording data and a cover layer for protecting the recording film L0 layer and the other layers below the recording film L0 layer are formed in an overlapping relationship with each other on a substrate of polycarbonate or the like. It is to be noted that an optical pickup (not shown) for irradiating a laser beam upon and receiving reflected light of the laser beam from the two-layer recording-reproduction optical disk is positioned at an upper location in the drawing. In the following description, where it is not necessary to distinguish the recording film L0 layer and the recording film L1 layer from each other, any of the layers may be referred to merely as recording layer.
In order to record data on a recording layer of a two-layer recording-reproduction optical disk, it is necessary to divide the recording layer into sectors of 2,048 (=2 K) bytes which are units for recording and reproduction and record a sector address into the header of each sector, that is, to perform a formatting process for the recording layer.
As a method of recording sector addresses and data on a recording layer, a method of forming pits (small holes) by stamping or the like in a process of production of a two-layer recording-reproduction optical disk and another method of irradiating a laser beam upon a recording layer of a completed two-layer recording-reproduction optical disk to record marks (phase transition regions) are known. It is to be noted that a pit formed in a process of production of a two-layer recording-reproduction optical disk is hereinafter referred to as embossed pit.
In order to record or read out a mark onto or from the recording film L1 layer of the two-layer recording-reproduction optical disk, a laser beam from the optical pickup is irradiated upon the recording film L1 layer through the recording film L0 layer or reflected light from the recording film L1 layer is received by the optical pickup through the recording film L0 layer as seen in FIG. 1.
Incidentally, a portion of a recording film at which an embossed pit or a mark is recorded has a different transmission factor or reflection factor with respect to a laser beam when compared with another portion of the recording film at which an embossed pit or a mark is not recorded.
Accordingly, when a laser beam is irradiated upon the recording film L1 layer through the recording film L0 layer or reflected light from the recording film L1 layer is received through the recording film L0 layer, a variation or an offset in amplitude occurs with the irradiation light or the reflection light in response to presence or absence of an embossed pit or a mark on the recording film L0 layer through which the laser light or the reflected light passes. Therefore, there is a subject to be solved that it is difficult to record or reproduce a mark with a high degree of accuracy onto or from the recording film L1 layer.